This invention relates to an electrolyte and more particularly to an electrolyte for use in a battery.
An effective solid electrolyte layer (SEI) must be created at the surface of a graphite negative electrode of a battery in order to keep the electrolyte from decomposing. Various electrolytes comprising certain combinations of salts and solvents produce SEI layers of various qualities. Typical lithium ion batteries use an electrolyte comprising LiPF6 in a carbonate solvent, with 1.2-M LiPF6 in ethylene carbonate (EC):diethyl carbonate (DEC) being typical in the battery industry. EC is solid at room temperature and requires additional processing steps for employing in an electrolyte. Graphite electrodes have a fragile structure and, until the invention of the electrolyte described herein, have required the use of EC for forming the SEI layer without damaging the graphite structure. By contrast, hard carbon negative electrodes are not as easily broken and therefore can use solvents other than EC to form the SEI layer. However, while hard carbon has a higher capacity than graphite, it can absorb a lot of moisture and has a large irreversible capacity, making graphite a much more desirable electrode material than hard carbon. Lithium metal does not require EC to form an SEI layer, but is useful only for a primary battery, not rechargeable. Vinylene carbonate (VC) and vinyl ethylene carbonate (VEC) can aid in creating an SEI layer, but can only be used in quantities up to about 3% because an excess of these solvents creates degradation at the positive electrode; with this small quantity of SEI-forming solvent, only a thin SEI layer is created, with all of the VC or VEC consumed during the first charging cycle; therefore, another SEI-forming component such as EC must be added.
The electrolyte of the present invention comprises a salt or mixture of salts comprising lithium bis(oxalato) borate (LiBOB) in a lactone solvent or mixture of lactone solvents, preferably gamma-butyrolactone (GBL), combined with a low viscosity solvent or mixture of low viscosity solvents, and preferably does not contain a solvent that is solid at room temperature, such as ethylene carbonate (EC). This inventive electrolyte is useful in primary and secondary batteries, and is especially suitable for a lithium ion battery having a graphite negative electrode, forming a functional SEI layer that does not readily decompose.
LiBOB is more soluble in lactone solvents, such as gamma-butyrolactone (GBL), than in commonly used carbonate solvents, such as ethylene carbonate (EC) and propylene carbonate (PC). Using a lactone solvent to dissolve LiBOB electrolyte produces a high salt concentration electrolyte, greatly improving conductivity as compared with using a carbonate solvent.
This electrolyte system has a wide operating temperature range and therefore can be safely used in many applications, including satellites and implantable medical devices. For example, a high temperature sterilization process could not be used for many electrolytes; the salt LiPF6 decomposes at about 80xc2x0 C., and DEC boils at about 126xc2x0 C. By contrast, LiBOB is stable at 300xc2x0 C., and GBL boils at about 206xc2x0 C., making this combination ideal for high temperature sterilization. At the other temperature extreme, EC has poor low temperature performance due to its high freezing point of around 37-39xc2x0 C., making it very viscous at low temperatures, and therefore less desirable for applications in which low temperature operation is important.
Furthermore, in the case of a leak, unlike fluorine-containing salts such as LiPF6, LiBOB does not form HF when mixed with bodily fluid, and is therefore safer than LiPF6. While LiBF4 decomposes at a lower rate than LiPF6 and is therefore slower to form HF, it has lower conductivity than LiPF6 due to its lower dissociation.